Since the development of various ceramic glasses in the 1960's and 1970's, the fundamental feature of extremely low coefficient of expansion has created the opportunity for smooth-top cooking surfaces with heating sources beneath the ceramic glass. Smooth-top cooking surfaces were attractive and practical because they were easy to clean.
Initially, the utilization of conduction heating of the ceramic glass plate, which in turn would heat the cooking utensil through contact conduction, was the only option as the ceramic glasses were largely opaque at all wavelengths. Although the thermal conductivity of the ceramic glass could hardly be classified as “highly thermally conductive” at about 2 Watts/meter-C.°, or less than one tenth the thermal conductivity of iron, conduction heating has been nearly universally implemented as the primary method of driving thermal energy through smooth ceramic glass cooktops to cooking utensils.
Techniques implemented when ceramic glass was introduced as a novel technology to the cooktop market were uncomplicated, such as that disclosed in U.S. Pat. No. 3,987,275, which was one of the first disclosures in which the ceramic glass was in physical contact with the heating elements. But techniques evolved quickly, such as that disclosed in U.S. Pat. No. 4,002,883, which employs film elements directly bonded to the ceramic glass.
Placing the heating elements directly in contact with the ceramic glass created opportunities for the elements to provide enough thermal energy to cause the ceramic glass to fail from excessive heat. To manage this situation, manufacturers tried to monitor the temperature of the ceramic glass and limit the thermal energy output of their heating elements as a safety measure.
Many patents were filed relating to monitoring the temperature of the cooktop as a means to control the temperatures of the heating element, the cooktop and the cooking process. Typical of these methods and apparatuses was U.S. Pat. No. 4,237,368, which discloses a method of bonding a thermistor to the ceramic glass in an effort to measure the temperature of the ceramic glass directly. U.S. Pat. No. 4,350,875 was one of the first to disclose a method of using an Inconel rod through a lever to activate a switch when the rod was heated and expanded over the hot radiant element. U.S. Pat. No. 4,430,558 discloses a similar apparatus and method to use an Inconel rod and switch to temperature-limit two radiant elements. U.S. Pat. No. 4,633,238 discloses a method of using a similar Inconel rod to directly activate a switch, eliminating the lever to lower costs.
There are methods and apparatuses to heat the bottom of the ceramic glass using heating elements bonded to the glass, where the heating elements themselves become part of the temperature measurement of the ceramic glass, as in U.S. Pat. No. 5,041,809.
Other U.S. patents, such as U.S. Pat. No. 6,111,228, disclose methods for using optical waveguide apparatuses as a means to capture infrared emissions from the cooktop in an effort to measure the temperature of the cooktop without making physical contact with the ceramic glass.
U.S. patents disclose new methods to control the thermal output of the heating element using analog Pulse Width Modulation, such as U.S. Pat. No. 5,565,123, or even microprocessor-controlled pulse width modulation, such as disclosed in U.S. Pat. No. 4,740,644.
U.S. Pat. No. 4,816,647 reveals microprocessor implemented methods to control the temperature of the ceramic glass, over-riding the user's selected heating rates if the temperatures of the ceramic glass exceeded safe levels.
Observations on the Early Years of the Ceramic Glass Cooktop:
Common to all of the disclosures related to the delivery of thermal energy to the ceramic glass cooktop and the efforts at managing the delivery of thermal energy by directly or indirectly measuring the temperature of the ceramic glass cooktop, is the assumption that the cooktop should be implemented as a conduction conduit for the thermal energy from the heating element to the cooking utensil resting on top of the ceramic glass cooktop.
To this end, all of the disclosures in the above referenced U.S. patents are a continuation of the functional features of the original ceramic glasses of the 1960s and 1970s: including limiting the temperature of the element to approximately 700° C. because the limiting operational temperature of the ceramic glass is 700° C.
The First Evolution:
New ceramic glasses were introduced in the mid-1990s with significant passbands for Infrared energy that exceeded 90% transmission at some wavelengths. U.S. patent disclosures after the development of the new ceramic glass technologies reflected changes in how the heating elements were constructed and used with the ceramic glass. U.S. Pat. No. 5,512,731 discloses a corrugated element that was set away from the ceramic glass. The corrugated element presented an expanded surface area that was largely perpendicular to the ceramic glass.
Still, heating efficiencies were very low and many efforts were made to limit the energy lost by the (resistive) element. The (resistive) element metal was perforated or configured to minimize the anchor attached to the ceramic insulator providing a mechanical mounting base. U.S. Pat. No. 5,699,606 discloses that the discontinuous means of the portion of the (resistive) element that is physically inserted into the mounting base would limit current flow and thus limit the thermal energy lost to conduction within the mounting material. U.S. Pat. No. 5,837,975 discloses a minimization of the amount of heating element material that is inserted into the supporting base, again in an effort to minimize thermal energy loss.
With the advent of the more robust ceramic glass, a corrugated (resistive) element with the major radiant surface set at right angles to the ceramic glass cooktop became nearly universal; but the means of managing the heating elements remained about the same. Various mechanisms were invented that either measured the glass directly or measured the thermal energy released by the heating element; but all measurements still feed a method to control an energy cut-off apparatus when the temperature of the heating source or the ceramic glass approaches the thermal limit of the ceramic glass, about 700° C.
This disclosure identifies four critical issues that were over looked or missing from similar systems in the market place and all related previous patents:                1. The highly transmissive passbands of the newer ceramic glasses are an opportunity to evolve to a more effective and efficient method of driving heat through the ceramic glass using appropriate radiant energy.        2. The wavelengths of the transmission passbands dictate the operating temperatures for the (resistive) radiant element using Wien's Displacement Law.        3. Application of the Stefan Boltzmann Law to the physical implementation of the temperature sensors and the construction of the (resistive) radiant element housing will increase system efficacy.        4. Constructing the (resistive) radiant element as a proper Lambertian Radiator will create a near optimum projected radiant pattern.The Highly Transmissive Passband:        
The introduction of the new generation of ceramic glasses in the mid-1990s should have generated a very large increase in capability and performance. By this time, smooth-top range and cooktop manufacturers had almost universally stopped direct conductive heating of the ceramic glass and implemented non-contact (resistive) radiant elements as their heating sources.
As can be seen in FIGS. 8 and 9, there are two passbands for infrared energy presented by the generation of ceramic glass introduced in the mid-1990s. (FIG. 8 shows transmission characteristics for non-tinted translucent ceramic glass; FIG. 9 shows transmission characteristics for opaque ceramic glass.) The wavelength vs. transmission plots are typical for the ceramic cooktop glasses popular in the marketplace and manufactured by either Schott Glass or Nippon Electric Glass, the two manufacturers which dominate this market space.
The abstracted charts show that the lower passband 420, 520 (low frequency, long wavelength) nominally covers wavelengths from about 3,500 nm to about 4,250 nm. The relationship of wavelength to temperature is given by Wien's Displacement Law:
  T  =            2.898      ×              10                  -          3                    ⁢                          ⁢              m        ·        K                    λ      peak      
These lower passband wavelengths correspond to temperatures of approximately 410° C. to 550° C. (about 770° F. to about 1022° F.), which is typical of the currently manufactured systems as reviewed by this inventor.
But as presented in the transmissivity charts, the peak transmissivity for the lower passband is at best 60%, and that is over a narrow portion of the band. This means that at the very best, radiant elements that operate in this lower passband are wasting at least 40% of their energy output as ineffective localized heating.
The upper passband 410, 510 (higher frequency, shorter wavelength) is characterized by wavelengths shorter than 2,700 nm and longer than 500 nm for clear ceramic glasses and for the heavily opaque second generation ceramic glasses from 2,700 nm down to at least 1,900 nm. These passbands, at wavelengths corresponding to temperatures between 800° C. and 1,250° C., are where the transmission of infrared radiant energy is nominally 70% to 90% efficient.
Those above-referenced patents which implemented temperature control of the heating elements of cooktop systems, whether the elements contacted the glass or not, universally disclosed that the upper limit of 700° C. was observed as a safety measure against glass failure.
A Higher Temperature Radiant Element:
The safe operating temperature of the glass should be observed, but the operating temperature of the radiant source should be significantly higher than 700° C. If the radiant source is operating in the upper passband, then the thermal energy transfer efficiency will increase by at least 10% and most probably by more than 30% over the actual operating range of temperatures.
The typical control system as related in the patents that were filed after the mid-1990s, as noted above, cuts off the energizing power to the elements when the energy radiating from the element is measured to approach 700° C.
A radiant source tuned to the lower passband with a maximum transmissivity of about 60% requires 100 Watts of transmitted radiant energy to deliver 60 Watts through the ceramic glass to the cooking utensil. What is worse is that for every 100 Watts of radiant energy directed at the ceramic glass, approximately 40 Watts will be lost to heating the ceramic glass.
Consideration of the transmissivity of the ceramic glass will significantly improve the operating parameters of the smooth ceramic glass cooktop. A radiant source tuned to the upper passband with the radiant transmission power of only 85 Watts will deliver approximately 60 Watts through the ceramic glass to the cooking utensil while only about 25 Watts will be lost to heating the ceramic glass. Both the overall reduction in power and the reduced loss into the ceramic glass improve the efficacy of the system. The ceramic glass can operate in the cooking zone longer and not get heated to the point of creating a safety concern. Overall energy is saved and operational costs are reduced.
The Stefan-Boltzmann Environment:
As noted above, there are several U.S. patents which have as a focus the improvement in the efficiency of the (resistive) radiant element. There were efforts patented that minimized the portion of the (resistive) element that was used to anchor the (resistive) element to the ceramic base.
In light of the Stefan-Boltzmann Law the concerns were unwarranted. The insulating refractory base used for providing physical mounting for the (resistive) element has a very low thermal conductivity and a very high thermal capacity. As such, the refractory in contact with the (resistive) element will quickly heat up and minimize the flow of thermal energy because the (resistive) element and the refractory anchor will quickly reach an equilibrium temperature.
In contrast are the attempts at measuring the temperature of the ceramic glass using attached but unshielded thermistors, unshielded contact sensors or optical waveguide temperature sensors. All of these considerations are confounded by the incorrect assumptions made relative to the use of a radiant energy sensor in the presence of the high-output radiant energy source (i.e., the (resistive) element) as compared to the energy emitted from the bottom of the ceramic glass plate.
The Stefan-Boltzmann Law defines the effectiveness of the radiant energy transfer as proportional to the 4th power of the difference in temperature. Given the Stefan-Boltzmann Law, any of these techniques to monitor the temperature of the bottom of the ceramic glass plate will be confused by the dominance of the high-temperature source.
Additionally, all “temperature” sensors measure “intensity” and not “power,” and as such they cannot differentiate between reflected, transmitted or radiated energy. Although the ceramic glass plate is only about 60% transmissive at 700° C., the optical characteristics of the ceramic glass would enable the “apparent” transmission of the radiant energy as indicated by the observed “intensity” through the ceramic glass. Thus optical sensors can be confused by their inability to quantify observed “power” and these sensors always find the highest temperature in their field of view, which could be a reflection of the radiant source.
An apparatus such as that disclosed in U.S. Pat. No. 6,111,228, using waveguides to “look” at the ceramic glass and duct radiant energy to an optical sensor, is unlikely to yield a reliable measure of the ceramic glass, because the higher temperature of the radiant source could be transmitted through the glass to the waveguide or reflected from the glass to the waveguide, dominating (by the fourth power of the difference) the lower-temperature radiant energy of the cooler ceramic glass plate.
Lambertian Radiators:
Lambert's cosine law defines how radiant energy leaves an emitting surface. All radiant surfaces that are not curved for some finite length are Lambertian Radiators. The corrugated (resistive) elements of the apparatus of several of the patents mentioned above are all mounted orthogonal to the ceramic glass underside, FIG. 7, 720. Unfortunately, these elements were constructed with their major surfaces placed at 90 degrees to the bottom of the cooktop. FIG. 7, with inset “FIG. 1” 740 excerpt from U.S. Pat. No. 5,512,731, shows the typical construction of the ribbon element in typical contemporary application. Blow up 730 reveals an enlarged view of the radiant surfaces of the (resistive) ribbon element. The large surfaces of the (resistive) ribbon element 710, 720 are placed at 90 degrees to the cooktop so that a very minimal amount of radiated energy is directly exposed to the bottom of the cooktop, as disclosed in U.S. Pat. No. 5,512,731 and others referenced above.
In fact, the only surface positioned so that it can directly radiate to the cooktop is the small edge of the (resistive) ribbon 700 which is at most approximately one tenth of the exposed surface area of the (resistive) radiant ribbon 730, leading to at best no more than 7% of the radiant output directed towards the ceramic glass cooktop with any potential to pass through the glass to the cooking utensil 3. Thus, the chief mechanism for heating from this type of thermal source is hot-air convection, which heats the glass plate and other supporting structures as a means to heat the cooking utensil on top of the glass plate.
These systems in manufacture and service over most of the world today are heating the bottom of the ceramic glass by convection air processes as the segments of the (resistive) radiant elements face each other over a significant portion of their length. As defined by the Stefan-Boltzmann law, the elements of equal temperatures will not effectively transmit energy to each other, but they will dramatically heat the air between them.
A radiant element operating as an effective Lambertian Radiator will project more than 70% of the total radiant energy emitted from the radiant element within a 45 degrees cone normal to the radiant surface.